The invention relates to a control system for controlling the power output of a laser diode.
More specifically, the invention relates to an integrated circuit of highly efficient construction which also compensates for slow changes in the output power vs. current characteristic of the laser diode. The circuit is particularly useful where the laser diode is modulated.
In laser printers, laser optical data storage systems, laser communication systems and other systems wherein a laser diode is employed, relatively slow changes occur in the output power vs. current characteristic of the laser diode. These can be caused by aging, varying temperature, or other changes in the diode's environment, or other factors. In a modulated laser diode the changes sometimes occur more rapidly or acutely, due to the repeated temperature changes related to the modulation pattern.
For dependable and continued accurate operation of the laser diode, there must be compensation for the relatively slow changes which affect output power.
Prior systems have included relatively complex circuitry for detecting and compensating for differences between actual diode output power and desired output power levels. The prior systems, which included feedback loops, often used voltage comparisons for determining and controlling drifts in power output, and this necessarily made the systems more complex and made it difficult to obtain a frequency response which would enable a faithful feedback signal.
None of the prior control systems was capable of dependably and accurately compensating for relatively slow drifts in a laser diode's output power vs. current characteristic utilizing simple bipolar circuitry included on a chip with very few external components, as in the present invention described below.